Soluble salts of mono-esters of sulfosuccinic acid have acquired importance as surface active agents. Conventionally such products are made by reacting maleic anhydride with a fatty alcohol, or an alkoxylated fatty alcohol or a fatty acid alkylolamide to form a half ester of maleic acid and this half ester is subsequently added to an aqueous solution of sodium sulfite to react with the sodium sulfite to produce the desired salt of the mono-ester of sulfosuccinic acid. The desired product is thus obtained in aqueous solution, and, if a solid product is required, it is necessary to dry this aqueous solution. This is usually done in a spray tower or on a film dryer.
It is a disadvantage that, in order to obtain the desired product in solid form, it is necessary to dry an aqueous solution, not only because of the cost of the drying operation, but also (and this reason is more important) because partial decomposition of the surface active substance occurs during the drying operation.
For the purpose of making a base for the manufacture of synthetic soap tablets, for which purpose the surface active agent used is required in a form having a low water content, it has been proposed in British Pat. No. 1,153,303 and German Offenlegungsschrift No. 2,157,634 to prepare a composition containing sulfosuccinates by reacting the maleic acid half ester (prepared in the conventional manner) with anhydrous sodium sulfite in the presence of a limited amount of water (usually in the range of 5-25%) and generally also in the presence of a waxy plasticising agent which remains in the product and becomes part of the synthetic soap tablet formulation.
Such processes yield products containing less water than the solutions of sulfosuccinate obtained by more conventional processes; but it has been found that, in such processes, the reaction of the starting material to produce the desired sulfosuccinate takes place only to the extent of about 50-60% of that theoretically possible, and moreover the end product has a high sulfur dioxide content (present as such or as a sulfite) amounting to 1.5-2.0% by weight of the product. Products made in this way may, on account of their comparatively low water content, be usable in the manufacture of synthetic soap tablets, but they are by no means entirely satisfactory in view of the low conversion of starting materials to sulfosuccinate achieved in making them; and in any event they are not conveniently used to make sulfosuccinate powders, as further drying of the product is needed for this purpose, and such drying cannot be carried out in a spray tower.